In media handling assemblies, particularly in printing systems, accurate and reliable registration of the substrate media as it is transferred in a process direction is desirable. In particular, accurate registration of the substrate media, such as a sheet of paper, as it is delivered at a target time to an image transfer zone will improve the overall printing process. The substrate media is generally conveyed within the system in a process direction. However, often the substrate media can shift in a cross-process direction that is lateral to the process direction or even acquire and angular orientation, referred herein as “skew,” such that it's opposed linear edges are no longer parallel to the process direction. Thus, there are three degrees of freedom in which the substrate media can move, which need to be controlled in order to achieve accurate delivery thereof. A slight skew, lateral misalignment or error in the arrival time of the substrate media through a critical processing phase can lead to errors, such as image and/or color registration errors relating to arrival at an image transfer zone. Also, as the substrate media is transferred between sections of the media handling assembly, the amount of skew can increase or accumulate. A substantial skew can cause pushing, pulling or shearing forces to be generated, which can wrinkle, buckle or even tear the sheet.
Contemporary systems transport a sheet and deliver it at a target time to a “datum,” based on measurements from the sheet leading edge. The datum can be a particular point in a transfer zone, a hand-off point to a downstream nip assembly or any other target location within the media handling assembly. Typically, the time of arrival of the sheet leading edge into a sheet registration system is measured by sensors located near the input of the registration system. A controller then computes a sheet velocity command profile designed to deliver the sheet at the target time to a predesignated datum. A sheet velocity actuator commanded by the controller then executes a command profile in order to timely deliver the sheet. Examples of typical sheet registration and deskewing systems are disclosed in U.S. Pat. Nos. 5,094,442, 6,533,268, 6,575,458 and 7,422,211, commonly assigned to the assignee of record herein, namely Xerox Corporation, the disclosures of which are each incorporated herein by reference. While these systems particularly relate to printing systems, similar paper handling techniques apply to other media handling assemblies.
Such contemporary systems attempt to achieve position registration of sheets by separately varying the speeds of spaced apart drive rollers to correct for skew mispositioning of the sheet, which is also referred to as differentially driven drive or nip assemblies. FIG. 4 shows the sheet registration system 8 from U.S. Pat. No. 5,094,442, which consists of two sets of drive nip assemblies 20, 30. As commonly referred to in the media handling art, each nip assembly 20, 30 includes a driven wheel 22, 24 (also referred to as drive rolls) and an idler wheel 26, 28, (also referred to as idler rolls) which together engage opposed sides of the sheet S and conveying it within the printing system in a process direction P. Also, included are separate drive motors and/or belt assemblies 21, 23 for imparting an angular velocity to the driven wheels 22, 24. While the motor may be connected directly to the driven wheels 22, 24, belts 21, 23, also referred to as timing belts, are often employed. Also, the motors may be stepper motors or DC servo motors with encoder feedback from an encoder mounted on the motor shaft, a driven wheel shaft or the idler shaft 25. The registration system 8 also includes sheet leading edge sensors 48, 50, which are used to detect the arrival of a sheet. The sequence of arrival at each individual sensor 48, 50 is also used to measure rotational mispositioning (skew) of the sheet. Temporarily driving two motors at slightly different rotational speeds provides a slight difference in the total rotation or relative pitch position of each drive roll 22, 24 while the sheet is held in the two nips 20, 30. That moves one side of the sheet ahead of the other to induce a skew (small partial rotation) in the sheet, opposite from an initially detected sheet skew in order to eliminate and correct for the detected skew.
FIG. 5 shows the sheet registration system 9 from U.S. Pat. No. 7,422,211, which also includes two spaced apart nip assemblies 20, 30 and a common idler shaft 25. As above, paper skew is corrected by a controller 60 prescribing differentially driven nips 20, 30 for a short period of time while the sheet S is engaged by the nips 20, 30. The sheet arrive time and skew are measured by sensors 48, 50 that are disposed along sensor line 41 that extends perpendicular to the process direction P. In such contemporary systems, while the nip velocities are varied, the average velocity between both nips must always equal the desired forward velocity of the sheet in order to maintain process speeds. In this way, both nip velocities deviate for a short period of time from the desired process speeds by the same amount, one being greater than the process speed and the other being less than the process speed by an equal amount. Also, the difference between the nip velocities will temporarily impart an angular velocity to the sheet used to correct skew. Thus, the resultant rotation of the sheet is always laterally positioned in the exact center between the two nip assemblies 20, 30. However, the position of that center of rotation is different from the lateral and process positions of either sensor used to detect the leading edge time of arrival. It is the leading edge time of arrive that is generally used, in conjunction with the registration distance D, to time the delivery of the sheet to the registration datum 100. In fact, when the sheet S is skewed one of the two sensors 48, 50 will detect the sheet S before the other. It is that first sensor time of arrival uses to time the arrival of the sheet S at the datum 100. However, the center of rotation, which becomes the corrected leading edge position after de-skewing, lags behind the initially detected leading edge. Accordingly, an error in the leading edge arrival time at the registration datum 100 is inherently introduced in such contemporary systems, unless the skew profile is known and further calculations are done to correct for the error. Also, such systems use a pair of symmetrically spaced leading edge sensors, which is limiting on the design configuration for print registration systems.
Accordingly, it would be desirable to provide a system for and method of accurately registering the leading edge of a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.